This invention relates to repairing cryogenic heat exchangers, especially those fabricated from aluminum. Such heat exchangers are conventionally employed in the liquefaction of natural gas and/or manufactured gases including those derived from petroleum.
In the various kinds of known natural gas liquefaction systems, aluminum is often the material of choice for the construction of the cryogenic heat exchanger routinely employed in the systems due to the high thermal conductivity, excellent low temperature properties, machinability and relatively low cost of this metal. However, aluminum is susceptible to corrosion by mercury which is ordinarily present in natural gas, e.g., from as low as about 0.005 to as high as about 220 micrograms per normal cubic meter (i.e., from about 5.5.times.10.sup.-3 to about 220 parts per billion by volume). Concentrations of mercury greater than about 0.01 micrograms per normal cubic meter are generally regarded as undesirable especially where aluminum cryogenic liquefaction equipment is concerned due to mercury's capability for forming a corrosive amalgam with aluminum. Even with the demercuration of natural gas prior to its introduction to the liquefaction equipment (see, for example, the demercuration processes described in U.S. Pat. Nos. 3,193,987; 3,803,803; 4,101,631; 4,474,896; 4,491,609; 4,474,896; and 4,500,327), a sufficient amount of elemental mercury will often remain in the post-treated gas as to pose a significant safety and maintenance problem where aluminum cryogenic heat exchangers are concerned.
Although of particular concern where aluminum cryogenic heat exchangers are involved, the safety and maintenance problems attributable to corrosive failure and/or stress corrosion cracking are common to cryogenic equipment fabricated from other materials, e.g., stainless steel, as well.
A variety of techniques for repairing damaged hollow articles such as containers, conduits, closed cooling systems, and the like, are known.
According to the method for sealing leaks in pipes and other rigid, hollow articles described in U.S. Pat. No. 3,556,831 to Schinabeck, et al., two epoxy resin compositions containing different curing agents are successively applied to the leakage site with the subsequently applied epoxy resin curing more slowly than the initially applied epoxy resin. The Schinabeck, et al. disclosure makes no provision for very low temperature application nor does it provide combination filling and sealing corrective procedure.
U.S. Pat. No. 3,608,000 describes a method for sealing leaks in pipes, conduits, gas-lines, closed containers, tanks, and so forth, in which the interior of such equipment is first purged of any moisture and/or oxygen which may be present followed by pressurized introduction of a sealant composition in an inert gas. The sealant composition escapes from any leaks present in the equipment into the outer ambient environment where it reacts with oxygen and/or moisture at the leakage site to form a solid reaction product. The sealant comprises (1) a volatile metal alkyl compound and (2) a volatile organosilane compound, e.g., a mixture of 20 volume percent diethyl zinc and 80 volume percent tetraethoxysilane in nitrogen.
U.S. Pat. No. 3,645,816 describes a method and apparatus for effecting the repair of fluid leaks such as those generally found around fasteners employed in the construction of rigid hollow articles such as aircraft integral fuel tanks. Repair of the leaks is accomplished by placing a preformed metallic foil patch having the same configuration as the protruding portion of the fastener and coated with a polysulfide sealant or other type of rapid cure sealants over the fastener. The sealant is then cured, preferably by application of heat to the localized area employing a heating tool having interchangeable tips for mating engagement with the particular preformed foil patch.
Damaged articles formed of aluminum or magnesium, including the alloys thereof, are repaired in accordance with the process of U.S. Pat. No. 3,711,310 by suitably preparing the damaged area as, for example, by grinding away the fault, then refilling the prepared area utilizing a plasma spray repair coating comprising aluminum or an aluminum-silicon alloy with a mixture of 10-40 weight percent molybdenum and, finally, refinishing to dimension as and if required.
As disclosed in U.S. Pat. No. 3,947,610, leaks in closed cooling systems containing an aqueous medium as the coolant are sealed by electroless deposition of a metal or metal containing compound from a water soluble, easily reducible metal compound. A complexing agent may be added to the coolant to assist in solubilizing the easily reducible metal compound. In one embodiment, the reducible metal compound is formed in situ by attacking the conduit walls of the cooling systems so as to form solubilized compounds, or compounds which can be solubilized by addition of suitable complexing agents.
U.S. Pat. No. 4,371,569 describes a method and apparatus for reinforcing and repairing piping in which a film is applied to the interior of the pipe. The film can be formed from a two-liquid synthetic resin paint which comprises a principal paint liquid and a solidifier liquid, a solid forming upon mixture of the two components.
U.S. Pat. No. 4,419,163 describes a one step method of applying a sealant to the inside surface of a pipe, such as a gas pipe, to repair any portions likely to cause a leak. The sealant is applied, then dried and finally solidified to seal the leakage sites. An injection means is provided for supplying the sealant in a cylindrical shape so that every portion of the inner surface of the pipe will be coated.
It is an object of the present invention to provide a process for repairing a cryogenic heat exchanger, especially one fabricated from aluminum, employed in the liquefaction of a gas such as natural gas.
It is a particular object of the invention to effect the repair of a cryogenic heat exchanger at low temperature, e.g., at or about the service temperatures which are typical of such equipment.
It is yet another object of the invention to provide a process for repairing a cryogenic heat exchanger working from an exterior surface, thereby facilitating both the repair operation and minimizing down time.